1. Field of the Invention
This invention relates to antennas and, more particularly, to a novel, inexpensive, and highly-effective antenna that has nearly constant gain over a hemisphere of solid angle so that it is essentially omnidirectional for antennas located near the surface of the earth. It is sensitive over a wide bandwidth and, compared to other inexpensive antennas, such as turnstile and patch antennas, has an improved impedance match and voltage standing wave ratio (VSWR).
2. Description of the Prior Art
For certain radio transmissions, circular polarization (CP) is desirable. CP is a special case of elliptic polarization in which the horizontal and vertical (orthogonal) components are of equal magnitude and exactly 90 degrees out of phase. Most polarized signals are not perfectly circular, but have some degree of ellipticity. References herein to CP include elliptic polarization in every possible range.
Turnstile, patch, and other types of relatively inexpensive antennas are known that are semi-omnidirectional--i.e., have nearly uniform gain over the celestial hemisphere seen from a point relatively near the surface of the earth--and have respective impedances that can be matched to those of the respective circuits in which they are used. Turnstile antennas are disclosed in a book entitled "Antennas" by John D. Kraus, McGraw-Hill Book Company, second edition, 1988, pages 726-731. A typical conventional turnstile antenna 10 (FIG. 1A of the appended drawing) comprises two dipoles 12 and 14 lying in a plane. Such an antenna is referred to hereinafter as a "planar turnstile." If the dipoles 12 and 14 are properly related to each other and properly driven and the plane defined by the dipoles 12 and 14 is horizontal, the turnstile antenna formed thereby can transmit or receive CP radiation very well at the zenith, which is directly above the antenna, but less well as the angle from the zenith increases.
Another well-known semi-omnidirectional antenna is commonly referred to as a "patch," or planar microstrip antenna. These antennas are also disclosed in the Kraus publication mentioned above (pages 745-749). With this type of antenna, the reduction in the vertical E-field component is even more pronounced, resulting in a severe loss of axial ratio for circularly-polarized signals in the plane of the horizon. A typical microstrip patch antenna is shown in FIGS. 1B, 1C and 1D.
An example of this effect is shown in FIG. 2. In this figure, where the angle is defined by a line from the zenith Z to the antenna 10 and another line from the antenna 10 to a point 16 displaced from the zenith, the component of the E vector in the vertical direction is reduced; and where the angle is 90.degree.--that is, where the angle is defined by a line from the zenith to the antenna 10 and another line from the antenna 10 to a point 18 on the horizon--, the vertical component of the E vector disappears entirely in the case of the patch and nearly so in the case of the turnstile, so that the radiation is no longer circularly polarized. Thus a conventional patch antenna and to a lesser extent a conventional turnstile antenna mounted with its base plane horizontal to achieve hemispherical omnidirectionality does not effectively radiate or receive circularly-polarized radiation to or from a region lying in a direction 90.degree. from the zenith. As FIG. 2 shows, the vertical component of the E vector decreases to nearly zero in this region. As the angle with respect to the zenith increases, the axial ratio deteriorates markedly, so that the conventional patch and turnstile are reduced to functioning essentially as linearly-polarized antennas.
In some applications, this loss of axial ratio (or reduction from circular polarization to linear) can mean a significant loss in system performance. For example, in the case where a signal from a navigation satellite is incident at a very low elevation angle above the horizon (80.degree. or more of off-axis angle from the zenith) on a receiver mounted on a marine vehicle, there are likely to be significant multi-path reflections from the surface of the water. When the receiving antenna is able to receive only a single, horizontally-polarized signal, it is likely that interference due to the multiple paths will induce severe fading of the signal, resulting in a loss of information. With an antenna that has good circular polarization (CP), however, the degree of fading is significantly reduced, since it is much harder to cancel out both the vertical and horizontal components with precisely the right 90-degree phase shift between the two signals. In other words, good CP vastly alleviates the problems of low look-angle reception.
Conventional patch and turnstile antennas moreover do not provide uniform gain over a solid angle of 180.degree. of celestial arc. Essentially constant azimuthal gain in the plane of the horizon is easily achieved by using two pairs of dipole elements arranged at right angles to each other. However, such an antenna provides more gain in a direction normal to the ground plane than in a direction parallel to the ground plane. This is a disadvantage particularly on moving vehicles (boats, for example) that exhibit roll and pitch in addition to yaw and translation and that need to transmit o receive omnidirectionally over the celestial hemisphere.
For example, consider a conventional patch or turnstile antenna mounted on a boat that is moored in quiet waters or is in a yard or dry dock. For best omnidirectional transmission or reception over the celestial hemisphere, such an antenna will be mounted with its ground plane parallel to the horizon and its mast extending in a direction normal to the plane of the horizon. The gain of the antenna will then be as shown in curve A of FIG. 3: namely, it will range from a typical maximum value at the zenith, shown in FIG. 3 as +5 decibels relative to isotropics (dBi), to a greatly reduced value on the horizon, shown in FIG. 3 as about -5 dBi.
Let it be assumed that this is satisfactory for reception of signals from, say, a navigation satellite that is anywhere above the horizon. Even on that assumption, reception of signals from a navigation satellite that is low above the horizon may be unsatisfactory at sea, where the boat is subject to roll and pitch. For example, suppose that the satellite is 90.degree. off the starboard bow and low above the horizon while the boat rolls to port. The ground plane of the antenna, which is fixed relative to the boat, will also roll to port, thereby correspondingly reorienting the curves of FIG. 3 so that the antenna gain will fall from the -5 dBi it provides when the boat is level (curve A, which relates to a conventional antenna) to a value less than that, which may be insufficient for adequate transmission or reception.
The situation is made worse when two boats communicate with each other using conventional semi- omnidirectional turnstile antennas. From time to time they will roll and pitch in such a way that the antenna masts tilt away from each other. In that case, the curves of FIG. 3 relating to the transmitting antenna will be rotated, say, clockwise, while the curves for the receiving antenna will be rotated counterclockwise. Thus a signal that is weaker because of the roll and pitch of one boat has to be detected by an antenna that is less sensitive because of the roll and pitch of the other boat.
Another problem with conventional patch antennas is that they are narrow-bandwidth devices that must be carefully tuned to achieve satisfactory operation at the desired frequency. This increases the complexity and cost of the impedance-matching tuning that is necessary to compensate for variations in materials, etc. A primary factor in getting a good SNR is the noise figure of the preamplifier. The antenna is usually tuned to get the best noise figure for nominal preamplifier impedance. But if the antenna has a narrow band, it is hard to guarantee that its impedance will be close to the nominal value at the correct frequency.
Another problem with conventional turnstile antennas is that separate mechanical and electrical structures are provided, thereby leading to undesirable complexity and unnecessary cost. In particular, the mast (mechanical structure) supporting the dipole elements and the driving balun (electrical structure) are physically separate, as disclosed for example in a patent to Counselman et al. U.S. Pat. No. 4,647,942.
Various attempts have been made to overcome the problems of conventional turnstile antennas noted above. The most notable is a drooping dipole arrangement disclosed by a patent to Woodward et al. U.S. Pat. No. 4,062,019. This device has radiating elements attached to mast at a 45-degree angle to the mast. The dipole elements droop down from their point of attachment in a straight line. The radiating element is thus at a 45-degree angle to both the plane of the horizon and a vertical plane through the mast. This inclination of the radiating elements makes it possible for the two orthogonal components of the electric field to exist over a much wider range of solid angle. In the case of planar patch and turnstile antennas (see FIGS. 1A-1D), the vertical component of E field in the direction of the horizontal plane (the ground plane) is significantly reduced as explained above.
So the Woodward et al. drooping turnstile antenna addresses some of the needs of a small, simple, semi-omni/CP antenna. Its most important characteristic is that the dipole elements are all straight lines, inclined at a 45+/-5-degree angle to the mast of the turnstile. In addition, the characteristic impedance of the drooping dipole is a fixed number that must be accounted for in the impedance matching network. (Naturally it is variable over a certain range dictated by dipole physical dimensions, spacing with respect to the ground plane, etc., but the range of variation is small.)
Other prior art of interest includes the following U.S. Pat. No.: 1,988,434, 2,110,159, 2,976,534, 3,919,710, and 3,922,683. However, no art heretofore developed discloses an inexpensive antenna that has essentially constant gain over a hemisphere of solid angle so that it is semi-omnidirectional, has excellent CP near the horizon, and is sensitive over a wide bandwidth and has an excellent VSWR.